One known way of connecting several radio transmitters to the same antenna or antenna line is to connect each radio transmitter through a separate bandpass filter having its center frequency tuned to the transmitting frequency of the radio transmitter. Such bandpass filters are called combiner filters. The function of the combiner filter is to feed the transmitting signal of the respective radio transmitter to the antenna with the smallest possible losses and prevent The entrance of transmitting signals of different frequencies from other radio transmitters in the direction of the antenna to this particular radio Transmitter as efficiently as possible. Conventionally, combiner bandpass filters are tuned fixedly to the transmitting frequencies of the radio transmitters. So it has not been possible to change the transmitting frequency of the radio transmitter without simultaneously changing the combiner filter or its tuning.
In certain cases, however, it is desirable that the frequencies of radio transmitters can be changed simply and rapidly. One such case is the base station of a cellular mobile radio system to which predetermined transmitting and receiving channels have been assigned. If the channel allocation of the system can be altered by varying the transmitting and receiving frequencies of the base stations, the channel capacity of the system can be utilized in a flexible and efficient way in varying conditions. Therefore combiner filters have been developed in which the center frequency changes automatically with the transmitting frequency.
The adjustment of known automatically tunable combiner filters is based on the measurement of the RF power reflected from the input of the filter or the RF power passed through the combiner filter, and locking to a minimum/maximum value of the measured power. A problem with this adjustment method is the low adjusting accuracy and the narrow dynamic range. As the frequency selectivity of the entire system relies on the combiner filter, power signal components of other radio transmitters leaking from the output of the combiner filter to its input cause the minimum reflection suppression value of the reflection suppression measurement at the filter input to be about 7 dB, which results in a narrow dynamic range for the measurement. In The adjustment method based on the measurement of the power passed through the combiner filter, the measuring dynamics of the maximum power value also remains low for the same reasons as have been stated above. In addition, the automatically tunable combiner filter based on this prior art adjustment method does not allow variation in the relative power levels between the radio transmitter, that is, the "mutual dynamics" is almost 0 dB, as a change in the power level of one transmitter affects immediately the power measurement in the adjustment circuits of the combiner filters of the other radio transmitters, thus causing adjustment error.